Cleaning an oil storage vessel is a difficult task because it requires removal of a layer of sludge remaining in the bottom of the vessel after most of the oil has been drained away. Sludge comprises mainly of a mixture of solids and oil and some water usually. Sometimes as much as 5 volume percent of the vessel is filled with sludge after the vessel has been drained of essentially all the oil stored therein. Typically, in a large vessel designed to hold in excess of 7,000 barrels of oil, up to about 1-5 volume percent of the maximum volume of oil held in the vessel prior to being drained remains in the residual sludge. This sludge consequentially is a source of volatile organic compounds (VOC). Prior to opening the vessel to the atmosphere (referred to as degassing in the oil industry) these volatile organic compounds need to be reduced, because regulations governing air quality standards have imposed restrictions on the amount of VOC present in the vessel prior to degassing. For example, the State of California's South Coast Air Quality Management District has adopted Rule 1149 governing “Storage Tank and Pipeline Cleaning and Degassing.” A draft staff report to the South Coast Air Quality Management District Governing Board discusses this Rule 1149. Rule 1149 requires that prior to degassing a vessel the level of volatile organic compounds remaining in the interior of the vessel be no greater than 5,000 ppmv, measured as methane, at least one hour after cleaning the vessel. Rule 1149 was adopted because using conventional cleaning techniques, although the VOC level was low immediately after cleaning, there remained volatile compounds in the sludge that gradually were released.
Moreover, cleaning and degassing such storage vessels is further exacerbated by the character of residual sludge in the vessel. Some vessels store water from an oil production well and the sludge therein contains little oil. Some vessels may include “sour” water because the water has absorbed hydrogen sulfide, mercaptans, or other sulfur containing compounds. Other vessels may retain a sludge containing a relatively large amount of “free oil” in the sludge. Some vessels may retain a thick layer of sludge that has little free oil and water and a relatively large amount of oil bound to solid sludge particles. An appropriate treatment regime is required for each of these different residual sludges.
This background discussion is not intended to be an admission of prior art.